As [1]} remarked, the width of the [O i] lines suggests that they are
also emitted at the centre of the ejecta, by the lowest-velocity material.
[1]} wondered how the same ejecta structure that produced Fe iii in
SN 1991bg can yield a low ionization and neutral oxygen lines (at a later
epoch) in SN 2010lp. We argue that the different mean molecular weight of an
oxygen-dominated gas (with mean molecular weight \(\mu \approx 16\) ) with respect
to an iron-dominated gas (\(\mu \approx 56\) ) would lead to a similar electron
density \(n_e\)  in the two gases even if on average only every other oxygen atom
released an electron in an oxygen-dominated gas, while every iron atom released
\(\sim 1.5\)  electrons (such that both Fe ii and Fe iii are present in similar
amounts) in an iron-dominated gas. Recombination depends on the product \(n_en_{ion}\) , where \(n_{ion}\)  is the number density of ions. Given that \(n_{ion}\)  in
a partially ionized oxygen-dominated gas is \(\sim 3.5\)  times as large as in an
iron-dominated gas, we may expect the ionization to be lower in the former at
similar mass density.
